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Streamflow and Nutrient Data for the Yazoo River below Steele Bayou near Long Lake, Mississippi, 1996-2000

U.S. Geological Survey Water-Resources Investigations Report 02-4215


by

M.S. Runner,
D.P. Turnipseed,
and R.H. Coupe


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ABSTRACT

Increased nutrient loading to the Gulf of Mexico from off-continent flux has been identified as contributing to the increase in the areal extent of the low dissolved-oxygen zone that develops annually off the Louisiana and Texas coast. The proximity of the Yazoo River Basin in northwestern Mississippi to the Gulf of Mexico, and the intensive agricultural activities in the basin have led to speculation that the Yazoo River Basin contributes a disproportionate amount of nitrogen and phosphorus to the Mississippi River and ultimately to the Gulf of Mexico. An empirical measurement of the flux of nitrogen and phosphorus from the Yazoo Basin has not been possible due to the hydrology of the lower Yazoo River Basin.

Streamflow for the Yazoo River below Steele Bayou is affected by backwater from the Mississippi River. Flow at the gage is non-uniform and varying, with bi-directional and reverse flows possible. Streamflow was computed by using remote sensing and acoustic and conventional discharge and velocity measurement techniques. Streamflow from the Yazoo River for the 1996-2000 period accounted for 2.8 percent of the flow of the Mississippi River for the same period.

Water samples from the Yazoo River were collected from February 1996 through December 2000 and were analyzed for total nitrogen, nitrate, total phosphorus, and orthophosphorus as part of the U.S. Geological Survey National Water-Quality Assessment Program. These data were used to compute annual loads of nitrogen and phosphorus discharged from the Yazoo River for the period 1996-2000.

Annual loads of nitrogen and phosphorus were calculated by two methods. The first method used multivariate regression and the second method multiplied the mean annual concentration by the total annual flow. Load estimates based on the product of the mean annual concentration and the total annual flow were within the 95 percent confidence interval for the load calculated by multivariate regression in 10 of 20 cases. The Yazoo River loads, compared to average annual loads in the Mississippi River, indicated that the Yazoo River was contributing 1.4 percent of the total nitrogen load, 0.7 percent of the nitrate load, 3.4 percent of the total phosphorus load, and 1.6 percent of the orthophosphorus load during 1996 - 2000. The total nitrogen, nitrate, and orthophosphorus loads in the Yazoo River Basin were less than expected, whereas the total phosphorus load was slightly higher than expected based on discharge.

TABLE OF CONTENTS

Abstract
Introduction
      Purpose and Scope
      Description of the Study Area
Methods
      Surface-Water Data Collection
            Remote Sensing Data Collection
            Unsteady Flow in the Channel
            Unsteady Flow in the Floodplain
      Nutrient Data Collection and Analysis
Streamflow
Nutrients
      Concentrations of Nitrogen and Phosphorus
      Loads of Nitrogen and Phosphorus
Summary
References
Appendix I -- Daily mean discharge hydrographs for Yazoo River below Steele Bayou near Long Lake, Mississippi, 1996-2000
Appendix II -- Maximum, minimum, mean, and 75th, 50th, and 25th percentile data for concentrations of nutrients in water samples collected in the Yazoo River below Steele Bayou, 1996-2000

FIGURES

1. Map showing location of data-collection sites in the Yazoo River Basin

2. Map of the vicinity of the Yazoo River below Steele Bayou near Long Lake, Mississippi

3. Cross sections showing velocity for the Yazoo River below Steele Bayou near Long Lake, Mississippi for full downstream flow, bi-directional flow, stratified flow, and full upstream flow

4. Graph showing relation between average velocity and instantaneous velocity used in discharge computations at Yazoo River below Steele Bayou near Long Lake, Mississippi, for data collected October 1996 through September 2001

5. Graph showing relation between river stage and cross-sectional area at Yazoo River below Steele Bayou near Long Lake, Mississippi

6. Graph showing instantaneous and daily mean velocity, and daily mean stage from September through December 1999 at the Yazoo River below Steele Bayou near Long Lake, Mississippi

7. Map showing location of high flow (stage greater than 91 feet) measurement sections at the Yazoo River below Steele Bayou near Long Lake, Mississippi

8. Computed daily mean stage and discharge from March through April 1997 at the Yazoo River below Steele Bayou near Long Lake, Mississippi

9. Graph showing concentrations of total nitrogen, nitrate, total phosphorus, and orthophosphorus, by month, in water samples from the Yazoo River below Steele Bayou near Long Lake, Mississippi, February 1996 through December 2000

10. Graph showing concentrations of total nitrogen, nitrate, total phosphorus, and orthophosphorus, with discharge, in water samples from the Yazoo River below Steele Bayou near Long Lake, Mississippi, February 1996 through December 2000

TABLES

1. Summary statistics for daily mean streamflow for the Yazoo River below Steele Bayou, 1996-2000

2. Concentrations of total nitrogen, nitrate as nitrogen, total phosphorus and orthophosphorus from water samples collected at three sites in the study area

3. Coefficient estimates and goodness of fit parameters for nitrogen and phosphorus load models

4. Results of load calculations for the Yazoo River below Steele Bayou, Mississippi, 1996-2000

5. Results of load calculations for the Yazoo River below Steele Bayou, Mississippi, for 1996 and 1997using models based on 2- and 5-year data sets




Water Resources of Mississippi


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